Question

Explain the following: There are two –NH₂ groups in semicarbazide. However, only one such group is involved in the formation of semicarbazones.

Answer 1

The two NH₂ groups has two nitrogen atoms that have unshared pair of electrons. The Nitrogen which lies adjacent to the carbonly group gets involved in bond formation with carbonly group.  In other words, the nitrogen closer to the C=O, gets resonance interactions with these groups and does not participate in bond formation. Therefore only one is involved in formation of semicarbazone.